1. Field of the Invention
The present invention relates to an apparatus and method for cleaning articles utilizing supercritical or near supercritical fluids. In another aspect, the present invention relates to a method for cleaning articles utilizing supercritical or near supercritical fluids and differences in solubility of the contaminant in the supercritical fluid at various temperatures, and to an apparatus for carrying out the above method. In still another aspect, the present invention relates to a method for cleaning articles utilizing supercritical or near supercritical fluids and differences in the density of the supercritical fluid at various temperatures to utilize convective flow in the cleaning process, and to an apparatus for carrying out the above method.
2. Description of the Related Art
It has long been known to use solvents in removing organic and inorganic contaminants from articles. In such processes, the contaminated article to be cleaned is contacted with the solvent. The contaminate is then solubilized by the solvent. Subsequent volatilization of the solvent separates the solvent and the contaminate. The vapors are then condensed and recontacted with the article to further clean it.
For example, U.S. Pat. No. 1,875,937, issued Sep. 6, 1932, to Savage, discloses that grease may be removed from the surface of metal castings and other nonabsorbent bodies by means of solvents.
One of the drawbacks of this type of cleaning process is that the cooling surfaces also have a tendency to condense water out of the atmosphere in addition to cooling and condensing the solvent. This condensed water then becomes associated with the solvent and thus comes into contact with the metal parts of the cleaning apparatus and with the material being treated.
U.S. Pat. No. 2,123,439, issued Jul. 12, 1938, to Savage, discloses that this problem of condensing water with the solvent may be overcome by first contacting the atmosphere with condensing surfaces at a temperature above the dew point of the atmosphere in which the operation is being carried out, but substantially below the condensing temperature of the solvent. The condensed solvent is utilized in the cleaning process. The uncondensed vapors are then brought into contact with cooler surfaces to condense out the water which is removed.
In addition to condensing the solvent on a cold surface and then contacting the condensed solvent with the article to be cleaned, it is also known to cool the article to be cleaned. For example, U.S. Pat. No. 3,663,293, issued May 16, 1972, to Surprenant et al., discloses that the degreasing of metal parts may be accomplished by generating vapors of a solvent from a liquid sump, establishing a vapor level by providing condensing means at the desired level, introducing the soiled cold part into the vapors, thereby causing the vapor to condense on the part. The condensate containing the soil falls from the parts into the sump. The part is taken from the vapor zone when its surface reaches the solvent vapor temperature.
In an effort to improve on the vapor degreasing methods, supercritical fluids have been utilized to clean contaminants from articles.
NASA Tech Briefs MFS-29611 (Dec. 1990), discloses the use of supercritical CO.sub.2 as an alternative for hydrocarbon solvents that are conventionally utilized for washing organic and inorganic contaminants from the surface of metal parts and machining fines. The typical supercritical cleaning process involves contacting a supercritical fluid with the part to be cleaned. The supercritical fluid into which the contaminant has been solubilized is then expanded to subcritical conditions to remove the contaminant. The cleaned fluid is then compressed back to supercritical conditions and contacted with the part to be cleaned. This cycle is continued until the part is cleaned.
U.S. Pat. No. 4,944,837, issued Jul. 31, 1990 to Nishikawa et al., discloses a method of cleaning a silicon wafer in a supercritical atmosphere of carbon dioxide. In the '837 patent, the supercritical carbon dioxide is first contacted with the silicon wafer to solubilize the contaminant. The fluid is then cooled to below its supercritical temperature.
Unfortunately, with the known processes of cleaning with supercritical fluids, the contaminants are removed with the fluid in a subcritical state. This means that energy must be expended cycling the cleaning fluid between the supercritical and subcritical state.
In addition, some of the prior art methods utilize forced flow of the supercritical fluid past the part to be cleaned to increase the effective cleaning efficiency. However, this forced flow adds cost in terms of energy requirements and sometime is detrimental when channeling occurs.
Therefore, there exists a need for a supercritical cleaning process in which the contaminants can be removed from the fluid while it is in the supercritical state. There also exists a need for a supercritical cleaning process not requiring forced flow of the fluid.